Field of the Invention
The present disclosure relates to a technique of misregistration correction in an image forming apparatus.
Description of the Related Art
One known example of an electrophotographic image forming apparatus is a so-called tandem image forming apparatus in which image forming units of different colors are provided independently. This tandem image forming apparatus adopts a configuration in which images are transferred sequentially from the image forming units of different colors to an intermediate transfer belt, and then the images are collectively transferred from the intermediate transfer belt to a recording material. In such an image forming apparatus, due to mechanical factors in the image forming units of different colors, misregistration (positional shift) could possibly occur when the images are layered. In order to correct the misregistration, in the image forming apparatus, detection patterns of different colors are formed on the intermediate transfer belt, and misregistration amounts in a main scanning direction and a sub scanning direction are obtained.
Japanese Patent Laid-Open No. 2001-356542 discloses a configuration in which a misregistration amount in the main scanning direction is obtained using a detection pattern with diagonal lines that form a certain angle with the main scanning direction. For example, a distance between two diagonal lines that together exhibit symmetry with respect to the main scanning direction, and a distance between one diagonal line and a line running in the main scanning direction, deviate from the ideal values due to misregistration in the main scanning direction. Therefore, the misregistration amount in the main scanning direction can be obtained by detecting the amount of this deviation. The length of such a detection pattern in the sub scanning direction increases in accordance with the maximum value of the estimated misregistration amount in the main scanning direction. The larger the length of the detection pattern in the sub scanning direction, the longer the time period of detection of the detection pattern. Furthermore, depending on the length of the intermediate transfer belt, detection patterns of all colors may not be placed on the intermediate transfer belt to fit within one revolution thereof.
In view of this, Japanese Patent Laid-Open No. 2009-053500 discloses a configuration in which the length of a detection pattern in the sub scanning direction is reduced with use of a detection pattern with two diagonal lines that are tilted in the same direction.
Provided that the maximum value of the misregistration amount that can be detected in the main scanning direction is the same, the length of the detection pattern disclosed in Japanese Patent Laid-Open No. 2009-053500 is two thirds, in the sub scanning direction, of the length of a detection pattern with a diagonal line that is tilted by 45 degrees with respect to the main scanning direction. However, it has been desired to further reduce the length in the sub scanning direction and detect the misregistration amount in the main scanning direction in a shorter period of time.